The present invention concerns a device for use in an aircraft fuel pressure sensor line. More particularly, but not exclusively, this invention concerns a fuel pressure sensor line connecting an aircraft fuel pump to a pressure sensor. The invention also concerns a fuel pressure sensor line reservoir for use with a sensor line connecting an aircraft fuel pump to a pressure sensor.
Large aircraft, such as the Airbus A380, include several fuel tanks, with fuel being stored in a number of fuel tanks located in the wings of the aircraft. In order to move the fuel from a fuel tank into an engine, or to move fuel between different fuel tanks during flight, an aircraft fuel tank may be provided with fuel transfer pumps. In order to be able to detect whether or not a fuel transfer pump is working, a sensor line may be connected to a feed line leading from the fuel pump outlet, the sensor line leading to a pressure switch associated with the sensor line. The pressure switch may comprise a diaphragm and a certain amount of residual air. When the pump is operational, the feed line pushes the fuel and water mix typically found in an aircraft fuel tank up the sensor line towards the pressure switch compressing the air in the switch. The pressure increase due to the fuel flow pressure, a typical example of which is 30 psi, activates the pressure switch and provides an indication that the fuel pump is working correctly. If the fuel pressure switch is not activated, a monitoring system may inform the aircraft operator that the fuel pump is not working, for example using a warning light and/or audible alarm.
However, the above described operation may cause problems. As the fuel pump goes through operational cycles of being on and off, water may build up in the pressure switch, potentially due to the greater density of water compared to aircraft fuel. The pressure switch may begin to act as a sump and collect the water. The water in the switch may then freeze and prevent the proper operation of the pressure switch. This may result in an indication that the fuel pump is not operating correctly, even when it is operating correctly. An investigation of the fault may cause the aircraft to be grounded, thus increasing the downtime of the aircraft, causing an airline operator a financial loss. The problem of pressure switches freezing may be most noticeable in the outer wing fuel tanks, due to the low relative height of the pressure switches compared to the fuel pumps. As an aircraft takes off and pitches to 18 degrees, any residual fuel/water mix that has remained in the feed line and sensor line can flow into the pressure switch where it again acts as a sump and retains the unwanted water, thus leaving the pressure switch vulnerable to freezing.
Proposals for overcoming the problem of freezing pressure switches include the following.
One proposed solution is to position the fuel pump and pressure switch such that the sensor line maintains a positive gradient during take-off, thus preventing the fuel/water mix travelling into the pressure switch as the aircraft pitch increases. Such a solution may be used in fuel tanks inboard of the outer wing tanks and pressure switches in such tanks have a lower failure rate than when this is not the case. However, such arrangements may not be feasible in existing aircraft, because of the cost of retrofitting such a solution, or because of the space restrictions in those aircraft fuel tanks.
An alternative solution is to change the route of the sensor line such that it maintains a positive gradient during takeoff. However, similar design restrictions as listed above apply when attempting to reroute the sensor line. Also, such a reroute may require the addition of a sharp negative angle into part of the sensor line, which could act to channel any condensation or water build up into the pressure switch.
Another alternative solution is to fill the pressure switch with Ethylene Glycol, which acts as an anti-freeze when mixed with water. However, this is a temporary solution only as the fuel/water mix will still enter the pressure switch and could eventually flush out the Ethylene Glycol.
The present invention seeks to mitigate the above-mentioned problems.